In various high performance optical imaging systems, A very large scale integrated (VLSI) complementary metal oxide semiconductor (CMOS) chip is manufactured on a silicon wafer by a sequence of material additions (i.e., low pressure chemical vapor depositions, sputtering operations, etc.), material removals (i.e., wet etches, reactive ion etches, etc.), and material modifications (i.e., oxidations, ion implants, etc.). These physical and chemical operations interact with the entire wafer. For example, if a wafer is placed into an acid bath, the entire surface of the wafer will be etched away. In order to build very small electrically active devices on the wafer, the impact of these operations has to be confined to small, well defined regions.
Lithography in the context of VLSI manufacturing of CMOS devices is the process of patterning openings in photosensitive polymers (sometimes referred to as photoresists or resists) which define small areas in which the silicon base material is modified by a specific operation in a sequence of processing steps. The manufacturing of CMOS chips involves the repeated patterning of photoresist, followed by an etch, implant, deposition, or other operation, and ending with the removal of the expended photoresist to make way for the new resist to be applied for another iteration of this process sequence.
The basic lithography system consists of a light source, a stencil or photo mask containing the pattern to be transferred to the wafer, a collection of lenses, and a means for aligning existing patterns on the wafer with patterns on the mask. The aligning may take place in an aligning step or steps and may be carried out with an aligning apparatus. Since a wafer containing from 50 to 100 chips is patterned in steps of 1 to 4 chips at a time, these lithography tools are commonly referred to as steppers. The resolution, R, of an optical projection system such as a lithography stepper is limited by parameters described in Raleigh's equation:R=kλ/NA,where λ represents the wavelength of the light source used in the projection system and NA represents the numerical aperture of the projection optics used. “k” represents a factor describing how well a combined lithography system can utilize the theoretical resolution limit in practice and can range from about 0.5 down to about 0.3 for standard exposure systems. The highest resolution in optical lithography is currently achieved with deep ultra violet (DUV) steppers operating at 248 nm. Wavelengths of 365 nm are also in widespread use and 193 nm wavelength lithography is becoming commonplace.
In production optical lithography, it is necessary to measure lens aberrations (e.g. localized shifts of focus or other imaging imperfections from the optical system) in order to ensure that the tools used in production will meet the desired quality requirements. Lens aberrations include defects such as defocus, astigmatism, coma, etc. It is preferable to obtain such measurements without actually producing product. During manufacture of such systems, lens aberrations are measured using techniques such as Phase Measuring Interferometry (PMI), and the optical system is adjusted until it is within tight specifications according to the PMI. If the lens aberrations are well-understood, the optical system can be made to compensate to some extent for such errors. However, after the optical system is complete, it is very difficult and uncommon to use PMI in the field to measure lens aberration. Additionally, lens aberrations may change over time, so that the corrections for lens aberrations measured during manufacturing become obsolete. Slight changes in lens shape or relative location become more significant at very short wavelengths, and for high NA optical systems. Therefore, at the level of performance currently required of lens systems for optical lithography, a robust in-situ method for simultaneous measurement of coma, astigmatism and higher order aberration is needed to support maintenance of optical lithography tool performance. A number of aberration metrology methods are known in the art.
For example, Dirksen et al. (“Impact of high order aberrations on the performance of the aberration monitor,” Proceedings of SPIE Vol. 4000 (2000), pp. 9-17) describes an in-line aberration measurement method, in which a circular pattern made of a pure shifter is imaged onto a photoresist layer. The presence of lens aberrations will make the printed pattern non-circular. The analysis uses a scanning electron microscope (SEM) to collect images and measure variations in critical dimension (CD). Since aberrations may be expressed mathematically as a series of polynomials, the SEM measurements are used to determine the coefficients of the polynomial series, thus providing a method of measuring the aberrations of the lens system. However, SEM measurements are not part of standard production operations and this method would require custom metrology equipment.
Farrar et al. (“In-situ measurement of lens aberrations,” in Proceedings of SPIE Vol. 4000 (2000), pp. 18-29) describes method for characterization of both the illuminator and lens by means of a diagnostic reticle. This method incorporates a Shack Hartman interferometer into a special reticle. This reticle is used to print a special pattern onto the photoresist, which is in turn measured using standard optical overlay metrology tools. One disadvantage of this method is that exposure times are much longer (up to 50 times) than normal exposure times. These specialty masks are non-standard, and susceptible to fabrication errors and misalignments.
Kirk et al. (U.S. Pat. No. 6,091,486 and in SPIE Vol. 3697 (1999), p. 70) describes an in-situ method of characterizing lens aberration by using a blazed grating, such that the interference pattern is asymmetric in amplitude. The zeroth and first order (asymmetric) diffraction components are imaged at different angular orientations through a range of focus to create a matrix of imaged areas on a resist where each image corresponds to a different combination of orientations of the blazed grating and focus conditions. The relief of each of the exposed areas within the matrix are compared using a dark field microscope system, and the variation in aberration is computed. However, this method requires the use of a customized microscope and special software for data acquisition.
In view of the foregoing discussion, there is a need to provide for a method for characterizing lens aberrations using standard production methods and equipment, using normal exposure dose and a single exposure set of data.